Enzymes transpeptidase

Figure 6.8 Pendllins are similar to the bacterial peptidogiycan terminal alanylalanine moiety. Because of this similarity, the enzyme transpeptidase recognizes 8-lactam antibiotics as substrate. As a result of this the 8-lactam is incorporated in the peptide chain thereby making peptide-peptide cross-linking impossible. The occurrence of this phenomenon stops the construction of the bacterial cell wall.

The enzyme transpeptidase cross-Unks the peptide chains of adjacent amino-sugar chains. 

The third and final stage of synthesis of cell walls occurs outside the cytoplasmic membrane. Thus, the transpepfidation reaction results in transformation of the linear glycopeptide polymer into the CTOss-Unked form. The enzyme transpeptidase, a membrane-bound enzyme, binds pentapepfide side chains by replacing terminal o-alanines. 

The high molecular weight (HMW) DD-peptidases feature two domains one of which is penicillin binding (PB). The subclass A contains bifunctional enzymes (transglycosylases and transpeptidases), and the subclass B, monofunctional enzymes (transpeptidases). Both the classes A and B of HMW DD-peptidases are essential for cell survival <1991ARM37, 1998MI1079>. Proteins of subclass C are involved in /3-lactamase synthesis induction (see Section 2.03.12.2) <1998AAC1>. 

Although their strained four-membered ring makes (3-lactam antibiotics susceptible to hydrolysis, this same elevated reactivity toward nucleophilic acyl substitution is responsible for their antibacterial properties. (3-Lactams act by deactivating an enzyme, transpeptidase, required for the biosynthesis of bacterial cell walls. The active site of... 

Ampicillin is a p-lactam antibiotic belonging to the penidllin group. Ampidllin is reactive with Gram-positive and some Gram-negative bacteria by inhibiting the enzyme transpeptidase required for cell wall formation. 

Study of the penicillin binding proteins has shown that bacteria contain several different enzymes (transpeptidases, endopeptidases and carboxy-peptidases) each involved with a separate aspect of peptidoglycan... 

Penicillin G inhibits the synthesis of bacterial cell walls, causing growing cells to burst. The enzyme transpeptidase catalyzes reactions that form the cell wall. The enzyme forms a complex with penicillin G, and the carbonyl group reacts with a serine hydroxyl group contained in the active site of the enzyme. The first step of the reaction is formation of a tetrahedral intermediate. 

Resistance can also arise when target enzymes, ie, the PBPs, and in particular the transpeptidases, are modified. Target-mediated cephalosporin resistance can involve either a reduced affinity for an existing PBP, or the acquisition of a supplementary, P-lactam insensitive PBP (139). 

The antibacterial effectiveness of penicillins cephalospotins and other P-lactam antibiotics depends upon selective acylation and consequentiy, iaactivation, of transpeptidases involved ia bacterial ceU wall synthesis. This acylating ability is a result of the reactivity of the P-lactam ring (1). Bacteria that are resistant to P-lactam antibiotics often produce enzymes called P-lactamases that inactivate the antibiotics by cataly2ing the hydrolytic opening of the P-lactam ring to give products (2) devoid of antibacterial activity. 

The biological activity of penicillins and cephalosporins is due to the presence of the strained /3-lactam ring, which reacts with and deactivates the transpeptidase enzyme needed to synthesize and repair bacterial cell walls. With the wall either incomplete or weakened, the bacterial cell ruptures and dies. 

This insertion is accomplished by an enzyme called transpeptidase. -Lactam antibiotics function as substrates for the transpeptidase, thereby establishing selective inhibition of bacterial cell wall synthesis. The structural similarity between -lactam antibiotics and the alanylalanine unit is remarkable as can be seen in Figure 6.8. 

A comparison of the structures of penicillin and Dalanyl-Dalanine (cf. structures 41 and 42) shows that there is a great deal of similarity between the two molecules. Penicillin is essentially an acylated cyclic dipeptide of Dcysteine and Dvaline (84). As such, it contains a peptide bond, that of the /3-lactam ring, that can acylate the enzyme. Labeling studies of the peptidoglycan transpeptidase of Bacillus subtilis indicate that radioactive penicillin reacts with a sulfhydryl group of a cysteine residue of the enzyme (86). 

Fig. 8.2 Interaction of transpeptidase (Enz) with its natural substrate, acyl-D-alanyl-D-alanine in the first stage of the transpeptidation reaction to form an acyl-enzyme intermediate. A similar reaction with a penicillin results in the formation of an inactive penicilloyl-enyme complex.

The antibiotic activity of certain (3-lactams depends largely on their interaction with two different groups of bacterial enzymes. (3-Lactams, like the penicillins and cephalosporins, inhibit the DD-peptidases/transpeptidases that are responsible for the final step of bacterial cell wall biosynthesis.63 Unfortunately, they are themselves destroyed by the [3-lactamases,64 which thereby provide much of the resistance to these antibiotics. Class A, C, and D [3-lactamases and DD-peptidases all have a conserved serine residue in the active site whose hydroxyl group is the primary nucleophile that attacks the substrate carbonyl. Catalysis in both cases involves a double-displacement reaction with the transient formation of an acyl-enzyme intermediate. The major distinction between [3-lactamases and their evolutionary parents the DD-peptidase residues is the lifetime of the acyl-enzyme it is short in (3-lactamases and long in the DD-peptidases.65-67... 

In an earlier report (J>), the decay of healthy yam tubers during storage was shown to be a result of catabolism of its proteins by an active a-glutamyl transpeptidase. There is also some alkaline proteolytic activity in the yam tuber (6), but little information is available on individual enzymes of the purine degradative pathway and on the properties of an alkaline proteinase that may function in yams during storage. This report describes the interrelation of five enzymes of ureide metabolism in fresh and stored yams, the release of ammonia in vitro by three of the enzymes that may provide an environment for alkaline proteinase activity in vivo, and the in vitro properties of an... 

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